1. FIELDS OF THE INVENTION
The present invention relates to a color liquid crystal display device having a single liquid crystal display element in which, after a plurality of beams in different wavelength ranges are projected to a single liquid display element respectively from different directions, these beams are combined to display a color image. More particularly, the invention relates to the one incorporated in a small-sized color television (TV) system of projection type and an information display system.
2. DETAILED DESCRIPTION OF PRIOR ART
Subsequently, generally known color liquid crystal display (hereinafter referred to as liquid crystal display) devices of projection type will be described.
As an alternative to a conventional so-called projection type TV in which an image displayed in a cathode ray tube is projected onto a screen, a projection type TV having a liquid crystal display element has been developed. Such a TV has already been commercially available. Since the liquid crystal display element is not self-luminous, it is required to provide a light source separately. However, the liquid crystal display element is, in principle, characterized in being capable of displaying as bright an image as possible in accordance with the brightness of the light source. Also, when compared to a projection type color TV having a screen of same size and using the cathode ray tube, the color TV using the liquid crystal display element can be manufactured with remarkably small-size and light-weight. Accordingly, there are great expectations for development of the liquid crystal display element in future.
A simple matrix liquid crystal display element and an active matrix liquid crystal display element are known, depending on the driving method thereof. The present invention can be applied to both types of liquid crystal display elements. Further, as a projection type color image display method using the liquid crystal display element, there are known a three-element display method in which three liquid crystal display elements corresponding to three primary colors are used and a single-element display method in which a single liquid crystal display element is used. According to the three-element display method, optical systems for transmitting respective color lights of three primary colors and display elements for forming an image by controlling respective color lights are provided in pairs, independent of one another. Images of respective colors are optically superimposed to one another to display a full color image. Unexamined Japanese Patent Publication No. 60-3291 discloses a display device which has three individual light sources and color filters serving as source emitting the respective color beams of red, green and blue. However, it is also possible that the light emitted from the single white light source is separated into color beams of three primary colors, i.e., red, green and blue, by dichroic mirrors. Subsequently, the respective color beams are projected onto the liquid crystal display elements corresponding to the respective colors. Such a display device is disclosed by the present applicant in Unexamined Japanese Patent Publication No. 60-169827. As a white light source, a halogen lamp, xenon lamp, metal halide lamp, or the like can be used. The emission spectrum of the white light source may be continuous spectrum or bright line spectrum. With this construction, since the light emitted from the common white light source can be effectively utilized, images are three times as bright as those obtained in the conventional single-element display construction to be described below, provided that other conditions are equal. However, more parts are required than the single-element display construction. Accordingly, in terms of cost and size, the above display device is generally disadvantageous compared to the single-element display device. Further, it will be noted that the dichroic mirror is made by forming a dielectric multi-layer thin film with a known thin film formation technique so as to selectively reflect or transmit only the light having wavelength to be found on a transparent substrate such as glass. Alternatively, the dichroic mirror may be combined prisms such as a dichroic prism used in an image pick-up device for a color TV, the surface of the prisms having similar dielectric multi-layer thin film for selecting the wavelength formed thereon. Hereinafter, both types of mirrors are merely referred to as dichroic mirrors.
On the other hand, in the former construction in which only one liquid crystal display element is used, similar to a direct vision liquid crystal TV, the light is projected on a liquid crystal display element having a color filter pattern of three primary colors in the form of a mosaic by the use of an optical system similar to a slide projector. Such a construction is, for example, disclosed in Unexamined Japanese Patent Publications No. 59-131278 and No. 59-230383. In the case where this single-element display method is adopted, the optical system can be simple in its construction and only one liquid crystal display element is used. Accordingly, the single-element display method is suitable to be adopted in a small-sized projection type system.
However, according to the single-element display method, about two-thirds of the light projected on the liquid crystal display element is absorbed or reflected by the color filter. For example, a red color filter is arranged on a picture element for displaying red. However, green and blue light are cut by the red color filter. Accordingly, only one-third of the light incident upon the liquid crystal display element can be made use of. Therefore, when compared to the case where three liquid crystal display elements are used with the same light source, the brightness of the screen is reduced to about one-third.
The conventional projection type color liquid crystal display device is summarily described above. In the single-display device, the white light is projected upon the liquid crystal display element and the color display is effected by arranging a micro dichroic mirror, micro-prism array, or diffraction grating for its corresponding group, each group consisting of picture elements of three primary colors. Such a construction is disclosed in Unexamined Japanese Patent Publications No. 61-210328, No. 62-293222, No. 62-293223, and No. 63-118125. However, it is, in reality, very difficult to form a micro dichroic mirror of about 100 .mu.m. Also, when the method is adopted for spectrally resolving the projected light through the use of a micro-prism array or diffraction grating, it is at present difficult to set an aperture efficiency of the picture element sufficiently high. Accordingly, it has been shortcoming of the above method that only a portion of the resolved spectral compositions can be made use of.